In recent years, as semiconductor device integration has increased, techniques for the reduction of the isolation region in semiconductor integrated circuits, which is an important element in miniaturization, have been actively studied. In particular, in memory devices of large capacities, the dimension of the isolation region is a major factor in determining memory cell size. The isolation region is necessary to prevent current flow between separate devices, such as transistors, that are located on the same substrate. However, since the isolation region does not play an active role electronically, its size should be minimized.
A conventional isolation technique is the LOCOS (LOCal Oxidation of Silicon) method, selectively growing a thick field oxide film on a semiconductor substrate and forming an isolation region. But, when the selected field oxide film is grown, so-called called bird's beak portions form from which extends the isolation regions towards the active element's regions, thereby expanding the dimensions of the isolation region. Therefore, expansion is pointedly a problem of highly integrated devices and techniques to reduce the bird's beak have been proposed, such as SILO (Sealed Interface Local Oxidation).
FIG. 1A to FIG. 1D illustrate the fabrication process sequence of an isolation region by using the conventional SILO method.
Referring to FIG. 1A, a first silicon nitride film 11 of 100.about.300 .ANG. is grown on a semiconductor substrate 10, and an oxide film 12 of 300.about.700 .ANG. and a second silicon nitride film 13 of 1000.about.2000 .ANG. are sequentially formed by a low pressure CVD method. Next, to define the active element's region, the first silicon nitride film 11, the oxide film 12, and the second silicon nitride film 13 are selectively etched by a conventional photolithography method, thereby forming an opening.
FIG. 1B illustrates the formation of a channel stop layer 14 by ion-implantation of the same conductive impurity as the substrate.
FIG. 1C illustrates the formation of a field oxide film 15 grown at a temperature of 1000.degree. C. to a thickness of approximately 4000.about.8000 .ANG..
Referring to FIG. 1D, the second silicon nitride film 13, the oxide film 12, and the first silicon nitride film 11, which were used to selectively oxidize the substrate, are sequentially removed, completing the isolation process.
However, in the conventional SILO isolation method, highly concentrated impurities are implanted to prevent the punch-through between active elements, which occurs along the bordering surface between the field oxide film and the semiconductor substrate. But, since impurity concentration is also high at the edges of the channel stop layer, adjacent to the high concentration regions, there is a disadvantage in that the junction breakdown voltage of the active element is lowered.